554 



COLLEGE ZOOLOGY 



Figure 399. A photograph to show variation in 

 growth. The pituitary giant is 20 years old and 

 8154 inches in height. The pituitary dwarf is 26 

 years old and 52 inches in height. (From Smiley 

 and Gould.) 



follicles; they contiol the menstrual cycle, 

 sexual behavior, and the development of the 

 accessory genital organs and secondary sex 

 characteristics. Progesterone is secreted by 

 the corpus luteum; it regulates the men- 

 strual cycle and plays a part in the develop- 

 ment of the mammary glands and in prepar- 

 ing the female for pregnancy. Relaxin, an- 

 other ovarian hormone, makes child birth 

 easier by relaxing the ligaments of the pelvic 

 girdle. 



Testis 



The interstitial cells of the testis secrete 

 a hormone, testosterone, which influences 



sexual behavior, and controls the develop- 

 ment of the male accessory genital organs 

 and the secondary sex characteristics. 



Pancreas 



The prancreas is a gland w^eighing two or 

 three ounces, situated behind the stomach. 

 The masses of cells in the pancreas, known 

 as the islets of Langerhans (Fig. 398), pro- 

 duce an internal secretion containing the 

 hormone insulin. This hormone is necessary 

 for normal regulation of sugar metabolism 

 it aids utilization of the glucose in the blood 

 it accelerates synthesis of sugar to glycogen 

 and it retards production of sugar in the liver 

 from fat and protein. If the pancreas does 

 not secrete a sufficient amount of insulin, 

 an excessive amount of sugar is present in 

 the blood, a condition resulting in the dis- 

 ease known as diabetes mellitus (sugar 

 diabetes). 



Gastric mucosa 



There is some evidence that the mucous 

 membrane of the pyloric end of the stomach 

 may produce a hormone called gastrin, 

 which is carried by the blood to the gastric 

 glands and stimulates the secretion of gastric 

 juice. 



Intestinal mucosa 



Parts of the mucous membrane of the in- 

 testine produce a hormone called secretin, 

 which was the first hormone discovered 

 (1902). Secretin is carried by the blood to 

 the pancreas, causing it to secrete pancreatic 

 juice at once. Another action of secretion 

 is to increase the flow of bile. A second in- 

 testinal hormone is cholecystokinin, which 

 causes the gall bladder to empty. 



Influence of hormones 

 on one another 



Finally, it is interesting to note that the 

 hormones secreted by the different ductless 



